1. Field of the Invention
The present invention relates to a variable speed magnetic medium recording and playback apparatus and, more particularly, to a variable speed magnetic reproducing apparatus.
2. Description of the Prior Art
In the field of magnetic reproducing apparatus such as a home video tape recorder (referred as VTR hereafter), an apparatus has been developed for performing a specific speed playback such as a fast playback, a still image playback, and a slow playback. In such apparatus, two rotary magnetic heads with different azimuth angles are used to alternately record or play back composite video signals of one field period. Video tracks with different azimuth angles are alternately formed on a magnetic tape, track each being aligned in the helical direction to the magnetic tape. The recorded video signals of one field period are alternately played back. For this reason, if video signals are played back while the magnetic tape is driven at a speed different from a speed in the recording mode, some video tracks can be traced over their entire length by one head with an appropriate azimuth angle. However, the other tracks often fail to be traced over their entire lengths by the head with the appropriate azimuth angle, or the other tracks may be traced by the other head with a different azimuth angle. As a result, levels of the playback signals from the other tracks are greatly reduced, thus causing noise on the screen corresponding to the other tracks.
Accordingly, a VTR including a specific speed, e.g., fast, still and/or slow, playback apparatus with a memory has also been developed. In this apparatus, only a composite video signal of one field, the record track, which is traced over its entire length by the head with the appropriate azimuth angle, is selected. Thus, the selected composite video signal, i.e., the selected field signal, has a sufficient level and is almost completely free from noise.
The selected field signal is converted to digital data by an A/D (analog to digital) conversion and the digital data is written (or stored) in the memory. The stored digital data is then read out from the memory and restored to the original composite video signal of an analog signal by a D/A (digital to analog) conversion to obtain a specific speed playback image on the screen of a video display device, which is almost free from noise. The VTR including the specific speed playback apparatus with such a memory uses neither extra heads nor any other means for intermittently driving the magnetic tape, and allows a simple arrangement for the specific speed playback. Such a VTR including the specific speed playback apparatus is, for example, disclosed by the Japanese Periodical "Television Technology", November 1985, published by Electronic Technology Publishing Inc.
However, when performing an extremely fast or slow playback at a speed of, for example, six times or 1/6 times the normal tape speed, the magnetic rotary heads inevitably scan across two or more of the record tracks so noise inevitably arises when the one azimuth magnetic rotary head scans the record tracks recorded by the other azimuth magnetic rotary head.
For example, considering a fast picture search at a speed of six times the normal tape running speed, the rotation speed of a rotary disc is controlled so that the horizontal synchronous signal of the playback video signals has a normal frequency. Also, the capstan servo system is controlled so that the phase of the playback control signal is locked at six times the rotary disc rotation speed. Therefore, the envelope of the playback video signals becomes one, as shown in FIG. 15, and a noise burr inevitably arises at the reduced level portions of the envelope. Accordingly, a specific speed playback image with less noise cannot be obtained with the apparatus described above.